You can see when an actor gets bored: Their eyes go dead. I promised myself I'd never let that happen. If it does, I'll go and live on a desert island for a year.
Now learning a bit more about footballers I think what they need to do well, is someone who really wants to stay in the background and just be a strong support.
And the most important thing - apart from telling a good, believable story, and being a true character - is to be someone the audience will care about, even if you're playing a murderer or rapist.