It's definitely good not to be jumping all over the place, constantly. However, you don't want to be too eerily calm during conversations. This may make people feel uneasy, or that you're not interested in what they're saying.
Instead, try to mirror the person you're speaking with. Don't mimic them — they'll probably get offended by that — but subtly copy some of their gestures and expressions.
Writing for Psychology Today, Dr. Jeff Thompson notes that mirroring will leave people perceiving you as positive and persuasive.
It can be tough to break out of your poker face, especially if you're just naturally not that expressive — but it's worth trying, since it can improve how you're perceived.