Yeah, I can see that happening. Meta gaming at its finest (sarcasm).
Perhaps this: First, Compelling a Talent may only ever affect one Test, either a single Test made by the PC or a single Test made by an opponent of the Player's choice, and only PCs (and NPCs with the Touched by Fate Trait) may Compell Talents. Most Talents are of benefit only to the PC who has them, so non-Ascencion Talents may not be Compelled to the purposes of benefiting another PC*. The duration of a Compelled Talent may vary, based upon the Talent/associated Skill description (see below). Next, when Compelling a Talent, the Player is already making a decision about which Talent to utilize, and those Talents typically provide an enhancement to Skills, and all Skills already have associated Characteristics. PCs must Compell a Talent prior to making a Test roll in order to benefit from any associated modifiers, and this Talent must be appropriate to the circumstances. Finally, a modifier that is assigned to or imposed upon Tests, positive or negative, will in all instances use the PC's unmodified Characterisitc Bonus1 as its maximum value. In other words, the maximum bonus/penalty will be no greater than unmodified Characteristic Bonus x1.
*Ascended Talents can possibly bestow benefits to other PCs within a pre-determined range or association, but as we're discussing individuals who have risen through the ranks of the plebes and ascended to the rank of Agents Imperialis, it's safe to say they have earned and are due a fate that is commensurately more significant and beneficial.
1This bonus/penalty modifer may seem small, perhaps insignificant, being in the average range of 3-4, perhaps 5 percentage points (or, on rare occasions, 6 or even 7), but I think it is exactly what you're looking for, just a little extra bit of fatum (fate, destiny, doom, lot) that the PC can actually call upon, and if the bonuses/penalties were much higher I suspect you'd find yourself wishing you'd never considered the idea. This will present more opportunities for a Player to role-play their PC to its strengths, perhaps more clearly defining their role within the Acolyte Cell. An example would be when Tech-Priests are permitted to substitute their Intelligence for their Fellowship when interacting with other adherents of the Machine God. I'm a moderate GM myself, and there are times when +/-10 is (to me) too steep a modifier. This mechanic is also in keeping with the value trends of Characteristic Modifiers (received during character generation as a result of "specialty"…or even the Imperial Divination Table) and Unnatural Characteristic modifiers used in Black Crusade/Only War, so it could easily be ported over to those games without "breaking" their mechanics.
If the PC is attempting to Compell his/her use of Air of Authority (Talent), that Talent indicates additional benefits by making a successful Command Test, and as Command is a Fellowship-based Skill the Player may add his/her Fel Bonus (along with any other modifiers assigned) to a single Command Test made to influence additional targets. Duration: Simple Command instructions require a Half Action, more complicated instructions require a Full Action, but in any case the Command Test is made during the PC's Turn.
If the PC were Compelling Hard Target (Talent); Hard Target makes it more difficult for an enemy to shoot you by moving faster/ducking/weaving, Movement is based on Agility, shooting is based on Ballistic Skill, so the Player could Compell his/her use of Hard Target to impose a negative modifier to one enemy's BS Test, and that modifier is equal to the PC's Ag Bonus. Duration: The (normal) penalty assigned to BS Skill Tests by Hard Target lasts until the end of the PC's next Turn, so at any time before then the Player may indicate he is imposing an additional negative modifier to any single BS Test made to hit him/her, but must indicate such before the Test dice are rolled (O God-Emperor, foul his aim!).
If the Player chose to affect his PC instead of an enemy, he would then receive a bonus to any one Dodge Test (an Ag-based Skill) he may need to make as an Evasion Reaction to being shot. Duration: One Dodge Test, until the end of the Round.
Or, Street Fighting (Talent); this Talent allows a PC to add +2 to the Critical Damage dealt by an unarmed attack or knife, Critical Damage dealt in this manner is the result of a successfuil WS Test, so the PC (sensing his opponent is "on the ropes") Compells his/her Street Fighting (Talent) and gains a positive modifier to a WS Test that is equal to his/her WS Bonus. Duration: One WS Test before the end of the PC's Turn.
*Finally, Heroic Leadership (Paragon Talent, pg.107, Ascension); Heroic Leadership replaces Air of Authority, Iron Discipline, Into the Jaws of Hell, and Master Orator. The benefits outlined for Heroic Leadership are based upon making a successful Command Test. An Acended PC might Compell his Heroic Leadership, receiving a bonus to his/her Command Test that is equal to his/her Fel Bonus. Duration: One Test (made in the PC's Turn).
I chose all of the above Talents at random, and it was very easy for me to consult the descriptive text of each and determine the associated Characteristic and duration. In all instances, the Player chooses which Test to affect, actively adding a fateful element to his PC's involvement within the story. In the case of imposing penalties to adversaries, it definitely benefits Players who pay more attention at the gaming table, as they can visualize events that are unfolding with more clarity and will have a better chance of being preemptive in their impositions.
So, now for the $47,000 question: How many fate Compelling points (I shall refer to them as Fatum Points) should PCs receive (at character creation, or retroactively in the case of a game already in progress), at what value should they be capped (if at all), and for what/how frequently are they awarded? Maybe that's three questions.
Here's my thoughts…
1I don't necessarily want a significant "game changer", just some "active" fatum for the PCs (and maybe a rare few NPCs). All new PCs begin with d5+5 Fatum Points. All PCs already "in play" (and NPCs with the Touched By Fate Trait) receive a number of Fatum Points equal to their unmodified WP Bonus. Will Power is associated with Psychic Ability, Psychic Ability is associated with the Warp, the Warp is associated with the unkown and the unknowable, and no one can know when fatum will bring inspiration or disincentive.
2I want the Players to know it's a dice mechanic benefit of role-playing cinematics. However, its intention is to promote better and more involved RP, not to add to the meta of game mechanics.
3Ten (10) is a number used throughout all FFG 40K RPGs, so 10 will be the cap, the maximum number of Fatum Points any PC or NPC may have.
4Fatum Points will be awarded for significant role-playing during social, intellectual and physical/combat circumstances (as determined by the GM, allowing for input on the part of the Players); the awarding of Fatum Points will not hinge upon successful or failed dice results. However, they will not be awarded following such a role-playing moment where Fatum Points were expended; the concept of pressing your luck with the result being you are equally (or more) lucky defeats the intention of this mechanic (see 2,above).
I know it's another wall of text (my apologies), but I believe it is both comprehensive and simple.
There is this: (Normal) Fate Points may already be "spent" to add an extra DoS to a successful Test, re-roll any one failed die result, count as having rolled a 10 for Initiative, instantly recover d5 Wounds, or recover from being Stunned. Ascension gets into more benefits, including gaining a +10 bonus to any one Test (chosen before the dice are rolled). It would be better to determine beforehand if, as a GM, you will continue to use Fatum Points as they are outlined above once the PCs have Ascended, or to ammend their benefits. I would opt for ammendment; I suggest removing the Ascended ability to add +10 to any one Test through the expenditure of a Fate Point, and increase the bonus/penalty value resulting from the expenditure of a Fatum Point to unmodified Characteristic Bonus x2. As with non-Ascended PCs/NPC, Ascended PCs/NPCs must spend the Fatum Point prior to the Test being rolled.
Then there is this: Per pg. 10 of the Dark Heresy Errata: "A Fate Point can be used at any time, either on the PCs own Turn or as a Reaction to the Action of another PC/NPC, and doing so is a Free Action." So, the spending of a Fatum Point will also be a Free Action. As there is no formal limit to the number of Free Actions a PC/NPC may take during his/her turn, it is left for the GM to adjudicate. However, for the purposes of Compelling a Talent, only one Fatum Point may be spent per PC/NPC Round.