If you examine the first chapter carefully, you can see that the relationship that is established between Pip and Magwitch is one that is based on fear and terror. Note that Pip is in the graveyard, contemplating the gravestones of his parents and siblings, when, zombie-like, Magwitch pops up suddenly from behind some graves:
"Hold your noise!" cried a terrible voice, as a man started up from among the graves at the side of the church porch. "Keep still, you little devil, or I'll cut your throat!"
Note the way that immediately Pip describes him as a "fearful man." His words and his threat straight away establish the way that fear plays a big part in his manipulation of Pip. Interestingly, as the description continues, we see the older, wiser Pip, looking back at how Magwitch appeared and seeing that although he was "fearful" he was almost pitiful in his appearance, as he was "smothered in mud, and lamed by stones, and cut by flints, and stung by nettles, and torn by briers..." However, the younger Pip is unaware of this, frightened for his life.
Magwitch uses the threat of tearing Pip's "heart and liver" out repeatedly to force him to bring him the food and file that he needs to free himself, frightening Pip dreadfully and manipulating him into stealing. Thus when we consider the relationship between these two characters, it is built straight away on fear and the threat of violence--themes that emerge as dominant in the rest of the novel.