Question: My impact character is my main character's long time friend. My protagonist is supposed to be a fairly decisive person, is there a way to create a balance between him being influenced by the impact character and sticking to his own judgement?Answer:
One way is to have your main character see
the impact character succeed at something by taking the opposite approach the main character would. You can also have the main character try to do things in his usual way and not achieve success. Or you could have the impact character beat the main character in some competition. Several incidents like these in the first two acts create pressure for the main character to change. Of course, the impact character could also try to persuade the main character to do things differently, but examples speak louder than words.
Usually, the main character won't actually switch to the impact character's way of doing things until the climax, when there's so much at stake he can't afford to fail. By that point, enough doubt has been created in his mind that he's willing to try something new.
Of course, some main characters refuse to change. In such cases, the impact character usually changes instead. Sometimes that works out, and sometimes it doesn't (depends whether you want a happy ending).