On the broader terms of this question: defining what is 'amateur' work, and 'professional' these days appears to me to be very difficult.
On the one hand, there are definitely certain members of the industry (generally of an older generation, who were brought within the system whereby Equity was a closed shop etc.) who still appear to feel that, if any work is performed which isn't Equity paid, then it is technically not 'professional' level work. This is clearly a contentious issue - because, whilst the level of payment may not meet Union standards, this doesn't necessarily mean that the production itself is stinting on its own production values. We can argue the toss (and frequently have!) on whether or not this is an acceptable working practice and actors should allow sizeable budgets to be spent on a project whilst they are deprived of adequate payment, but as a purely aesthetic issue, matters are more complicated. Although it is an easy thing to say that all projects which fail to pay an actor adequately are necessarily below par and 'amateurish', this isn't always true. Indeed, such is the state of the modern industry, with the closed shop finished with, with everybody endlessly cutting wages right, left and centre, with the ability to be seen by casting directors/agents and making industry contacts ever more difficult and inflexible, and increasingly dependent upon appearing in fringe productions etc., that the whole question of what is definable as 'professional' work seems to have become totally fluid. One can seriously ask the question: is working with, say, a theatre company which pays regularly but produces sub-standard fare for the audience actually more or less 'professional' for your career than taking part in a deferred pay feature film, which is seen internationally, gains limited release and attracts 'name' actors to the cast? If you are basing a distinction solely upon the economic details, then you have your answer...but this is only one form of distinction within an industry within which the actor frequently has to take work wherever they can find it.
The most workable distinction I can come up with is that working in 'fringe' theatre, lo-budget film etc. is a 'grey area'. We actually call 'fringe' theatre that, to distinguish it from 'amateur' productions, because although the pay is (mostly) non-existent, it is accepted that this kind of production is now prevalent within the industry. 'Fringe' generally takes place in specific theatrical venues (many of which are known and recognised), and invariably features actors who are professionally trained or intending to become professionally trained in the near future. It is used as a scouting ground for agents and reprsentatives etc. etc. regardless of the fact that we all know some 'fringe' is brilliant, and some is dire.
Pure 'amateur' theatre, I would say, is basically defined by the fact that the people who take part in it a) play very much to a circumscribed audience i.e. they don't truly promote to the wider population b) don't ever suggest the possibility of making money from a venture - profit share, as we all know, hardly ever works, but it is a theoretic agreement that the actors are in a production, at least in part, 'for the money' and c) most importantly, are not considered by themselves or others to be actors by profession. This is wholly different, psychologically, to the notion that many working actors have to have a day job to make ends meet. All this means is that they consider themselves actors first and foremost, who aren't being paid enough to keep body and soul together. Someone who is actually earning a nice living as a travel agent, or a school teacher, and acts at weekends for the 'fun of it' is clearly not the same sort of person at all, and we can all recognise the distinction. When you get right down to it, professionalism is defined separately from the 'amateur' by the fact that you have committed body and soul to the fact that acting is a JOB OF WORK, that must be pursued in whatever way it can be (and whether you are being paid the right rates for it or not!), not something to be done as and when the fancy takes you.
