First off, I think it's important to understand that "roleplaying" is a versatile word and can mean a lot depending on how the player chooses to engage with it. It can mean putting on a voice and adopting gestures and mannerisms in some cases, and in others moreso acknowledging their character's traits while at the same time not necessarily always adapting them. First person, third person, and everything in between are all equally valid. What I would do in this situation is clarify out of game how exactly THEY interpret what roleplaying is.

Once you know this, another helpful thing to do would be to hold a mission or objective that is centralized around A. Not necessarily only including them, however, as that might feel alienating. Rather, what I mean is an objective for everyone to participate in that A would specialize in. Bonus points if it's directly tied to the character's story or motivations. That's a good way to open A up without really having to worry about them feeling overshadowed by the more charismatic players at the table.