Spectator, observer, beholder, looker-on, onlooker, witness, eyewitness, bystander, kibitzer are comparable when they mean one who sees or looks upon something.
Spectator can be used precisely in place of auditor for one that attends an exhibition, performance, or entertainment which does not involve an appeal to the sense of hearing; thus, one tends to speak of the spectators at a football game, a prizefight, a pageant, a pantomime, or a circus but of the auditors or the audience at a concert, a lecture, or a play. Very often, however, the term is used more broadly to denote one who regards himself or is felt to be wholly apart from and in no manner identified with what is presented to his attention (as by his sight, hearing, or understanding).
Observer may or may not imply an intent to see, but it usually does suggest, whether one sees by intention or by accident, that one attends closely to details and often keeps a record of them; the term applies especially to those (as scientists) who gather evidence by carefully noting phenomena or the results of experiments or to military or diplomatic officials who are sent by countries not participating in a war or in a meeting of representatives from other nations to watch proceedings closely and to make a report of them; it is also applicable to whoever has formed similar habits and can be more or less relied upon for accuracy.
Beholder sometimes carries a stronger implication of watching or regarding intently than either of the preceding terms, but it may mean little more than one who sees. The term is often applicable to one who has been privileged to look intently upon or, sometimes, consider deeply a person or thing with the result that he obtains a clear and accurate impression of that person or thing and is moved by the qualities (as beauty, power, tenderness, or pathos) of what is seen.
Looker-on and onlooker differ from beholder chiefly in their suggestions of casualness or detachment and in their definite implication of lack of participation. Either term is sometimes used in place of spectator when the distinction between the one who sees and what he sees is stressed.
Witness specifically denotes one who has firsthand knowledge and therefore is competent to give testimony. The term sometimes applies to a person who knows because he has seen, but since witness does not necessarily imply seeing, eyewitness is often preferred as more explicitly implying actual sight.
Bystander primarily denotes one who stands by when something is happening; sometimes it carries the implication of onlooker , but at other times it suggests little more than presence at a place.
Kibitzer specifically applies to one who watches a card game by looking over the shoulders of the players and who may annoy them by offering advice; in extended use the word denotes an onlooker who meddles or makes unwelcome suggestions.