Thank you Alophysio for your reply, very usefull.
I would like to take one step further back. Just go with the patient to the gym and look how he performs. Or just do a thorough observation in "resting postion" and in action. Do not focus on the glenohumeral joint. I have had a patient with a clunk which disapeared when mobilising the thoracic spine. Movement is about balance between muscles, joints etc. Activity of muscles is depending on (to a larger degree) the nervous system which puts things in motion. Therefor any false imput by the nervous system will effect movement.
Stretch as Alophysio mentioned in his example could alter the muscle tone and restore the right balance.
Look at things like internal rotation of the arms (quite likely thumbs pointing inwards or even backwards, while normally they should point forwards in rest) this means an overactivity p.e. pectoralis major.
Decreased thoracic movement will effect scapula movement. (extreme shoulder movements are accompenied by high thoracic rotation)
I meant to give just some examples so you understand why I say; back to the drawing board.