difference between supporting, main, and principal role in acting?

"Main" role is just a generic term used to indicate that the role has a fairly large part in the show as opposed to someone in the background or someone who only has a couple of lines. Both a supporting and lead role would be a "main" role. There are times where a "principal" role would NOT be a main role (see below). I can't think of an example where someone would be thought to have a "main" role but not be the lead, supporting or principal role. I guess if they were a "guest star" or something that could be the case.

When writing an acting resume the word "main" would not be used. Lead, Principal, Supporting are terms used. Based on the situation, they have specific meanings. Basically they're ways that the union categorizes roles.

A principal role (for union purposes) is basically any role that has a speaking line. That includes a U-5 (under 5 lines) role or a lead. The lead is the character which the story pivots around - there wouldn't be a story without the lead. If there is more than one lead then you can use the term "co-lead". Supporting character is the character in the story that lends support to the lead. Often when you sign a contact to be in a show the type of role (principal, lead, supporting, guest star) will be spelled out in the contract.

Hope that helps.

Most movies/TV shows/plays have one, two, three or four leading roles, which are also called principal or main roles. That's the same thing.

The less important roles are called supporting roles. That doesn't mean that they 'support' the lead characters literally. But it covers all the main characters, the ones with names and a fair amount of action and/or lines.

Much smaller roles played by well-known actors are known as cameo appearances.

Different people use different terms. 'Lead' and 'Supporting' and the most common.

The leads have the most lines. Supporting have less lines, and i'm not too sure about principal