I usually just do my sets and leave. I spotted for a guy benching once, and he was clearly having some trouble locking out the reps. I told him that dips and creatine might help. That's about the only time I've ever given advice to anyone at the gym. If people want to do half-squats in the Smith Machine, that's their business.
In matters of style, swim with the current. In matters of principle, stand like a rock.
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