Jazz and the fine balance of performing

Last night we went to a super cool jazz club in Dalston, where a jazz trio was performing (perfect Friday eve). At some point, the bass player went into a solo, and by went I mean "really went for it" - performing with close eyes, body moving with the music, and the very expressive face of someone transported by music.... The music was beautiful indeed, but the question on our lips was: is he faking it?
When I practice the flute, my body doesn't really move - I concentrate on my breathing, the technique, and my fingers' rapidity. However, when I perform for someone I always find myself moving my body much more with the melody. To be honest, it is a bit to show off... but it also helps me feel the melody much more and be more musical.
And then I always reach a point where I feel that the body movements overtake the technique and my sound gets weaker - then I need to tone the "performing" act down.

I was thinking last night that acting is similar - we feel the emotion, then we push it out for the audience to see, which makes us feel it even more and gives strength to our movements... until the point where we feel that we just overacting and losing the connection between performance and feelings. Then we need to find the fine balance between "feeling in" and "showing out".