A) two actions (the main one will be the one we use the tensed verb for and the secondary one, the one in participle form):
3. Jack stared at her, wondering what to do next.
B) two states, or an action and a state (usually, we will keep the main verb for the action and reserve the participle for the state that provides background information). In your sentence, although "standing" describes a state of stillness, that is, a stationary position, it is in fact an action verb, while "looking" is a stative verb, so the rule I just came up with applies:
1. Jack stood there alone, looking surprised (with a look of surprise on his face).
We can also have a state verb as the main verb followed by an action verb, but without a comma — the action verb will account for the state, for example:
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