Bryan Cranston’s defence of playing a wheelchair user in the new comedy-drama The Upside has underlined a particular point in the complicated issue of equality and diversity in acting: should non-disabled actors play disabled roles?

We are familiar with the repellent days of “blacking up” – the disabled actors’ equivalent is “cripping up”, a term used by acting activists to highlight that it is not acceptable for a non-disabled actor to mimic impairments, then win an Oscar.

The acting union Equity has said that in casting “disabled” roles, “every avenue” should be considered to cast a disabled actor. Yet the challenges for disabled actors and the representation of a disability experience in film are not isolated to casting. Fundamental barriers to auditioning limit spaces for disabled actors; for those who do get work, it is still mostly for disabled roles written by non-disabled writers, which may present stereotypical or unrepresentative characters.

There is a need to think critically about how disability is being defined, writes academic Dr Alison Wilde in her book Film, Comedy and Disability, “with representational and employment concerns deeply interwoven into the film industry’s ideologies, practices and processes … There are dangers we may thwart our own goals.”

The result of this self-perpetuating cycle is limited opportunities to tell the disabled story in all its dimensions. Disabled actor-writer Mat Fraser, who played “crippled” Richard III last year, summed it up: “Ideally, anybody should be able to play any body, but only when there is a truly level playing field of opportunity.” Every theatre should commission at least one disabled playwright and cast at least one disabled actor each year, he wrote, as a condition of arts funding.

Don’t tell me there are no disabled actors with talent. This week, I watched Silent Witness with Liz Carr, a disabled actor who, for seven years, has played disabled lab technician Clarissa Mullery. Carr, an alumni of Graeae Theatre Company, was excellent as usual, yet here’s the rub – she only got a handful of lines on Monday.