The actors’ union Equity has called for more casting directors to consider hiring transgender performers to play non-trans characters.

In guidance for entertainment professionals working with LGBT+ performers, published on Monday, the union said some trans actors were better suited to playing characters who were cisgender – meaning their gender identity matches the sex they were assigned at birth.

“The fact that [a performer] is trans may be completely invisible in the role or production, but it powerfully represents diversity in the industry,” it said. “This ‘invisible’ diversity is just as important as more physically recognisable forms of diversity.”

It added: “It is hard for trans actors to build a career out of the very small amount of trans-specific roles if these are the only roles for which they are actively sought.”

At a launch event for the guidance at the National Theatre, Tigger Blaize, the vice-chair of Equity’s LGBT+ committee, said the advice was designed to “help you to help us”. “We don’t want to bash anyone,” he said. “This is a new area to many and it’s OK to make mistakes.”

The union’s guidance says it should be standard industry practice for people to introduce themselves with both their name and preferred pronoun. It advises asking performers what pronouns they prefer, rather than assuming. “If you make a mistake, apologise, correct and move on. No dramas,” it reads.

Last year two films made headlines for casting trans actors to play cisgender characters. Rebecca Root played Mayfield in The Sisters Brothers, and Jake Graf played Gaston Arman de Caillavet in the film Colette, starring Keira Knightley.

Harrison Knights, a trans actor and activist, said he sometimes struggled to get parts playing transgender characters. “I’ve got a beard and a low voice, which they’re not expecting,” he said. Playing cis roles was “the holy grail” for him, he said, because there were more of them and they were more diverse.

Knights said cis roles were “the assumed default”. “An unnamed drug addict on Casualty, we assume he’s cis. A trainee barrister on Silk, we assume she’s cis. In fact, we assume everyone is cis unless told otherwise.”

He added: “It is not until [trans actors] are being cast in major cis roles because we are the best actors for the role, rather than because we tick a box, that we will have truly arrived.”

Knights said 48% of trans young people had attempted suicide, and having more visible trans role models would help this situation.

“This may be [achieved] through trans storylines – and these have given good exposure on TV shows – but more sustainably, and somewhat more bravely, it should come through reading for the good visible cis parts that are often out of our reach,” he said.

Blaize, who in more than 15 years of being an actor has played cis female and male characters, said he hoped the guidance would be empowering. “We are in the middle of a really horrible media storm of misinformation about us,” he said. “There are horrible stories coming out from hate groups and we need some positive representation.”

• This article was amended on 30 October 2019 to correct the spellings of Keira Knightley and Jake Graf.