I think you're right that he seemed to be more of a follower - so stronger personalities like Philby and Burgess could sway him one way or the other. And then when he was too far gone to leave "the game", he realized what happened (I"m only speaking about the movie here, mind you - I think IRL they were all quite different characters from how they were portrayed ...)

Anyway, of course, I very much love the entire series, but it's mostly Toby for me in "Cambridge Spies." ;)