I am writing a one act play about a trial. The evidence is piled against the defendant, and he wants to plead insanity. What are other ways of stating that? What are other ways to refer to that plea? I do not want to be overly repetitive by using only one phrase.
I was a prosecutor in the US, which does not mean I am right, but it might affect how you interpret my opinion.
First, "plea of insanity" is imprecise. In all US jurisdictions, when the defendant is required to enter a plea, the defendant may plead guilty or not guilty. Some jurisdictions allow a plea of no contest.
Insanity is an appendage to the above pleas. Most jurisdictions allow a defendant to enter a plea worded something like "not guilty by reason of insanity." A few jurisdictions allow a plea similar to "guilty but insane." The root issue of insanity in a criminal charge is the culpability of the defendant. If you believe it is necessary to not be "overly repetitive", then if you study the legal theories of culpability, then you will find much inspiration.
Nevertheless, legal terms such as "not guilty" and "insanity" are extremely precise, and when writing and speaking about law using synonyms for legal terms-of-art is highly problematic. I realize you are writing a play and not a legal memo, but consistently using terms of art will reduce ambiguity and confusion, so you may want to consider not using a synonym except when the speaker does not have legal training.
Off the top of my head, the only substitution that criminal lawyers might make is to say, "The defendant is claiming insanity." In that sentence, "claim" is a term-of-art that is an acceptable synonym for plea.
Good luck with your play. As a former trial lawyer, I don't envy your challenge of making a trial interesting to spectators!
In the UK all this is largely governed by The Criminal Procedures (Insanity) Act 1964 which replaced the Trial of Lunatics Act, 1883.
A number of possibilities are open. A person can be found to be unfit (by virtue of insanity) to plead. If such a condition is accepted by a judge, the trial simply does not take place. one key question can be as to whether the defendant possessed mens rea (an intending mind) at the time of the alleged crime.
They can also be found not guilty by virtue of insanity, or guilty but insane.
There are other possibilities. It is a complex area of the law.
There is a set of rules known as the M'Naghton Rules which were those applied in the celebrated case where Daniel M'Naghton was acquitted in 1843 of murdering Edward Drummond (whom he mistook for the Prime Minister, Robert Peel). In Britain and other Common Law jurisdictions the M'Naghton Rules still provide guidance to judges in determining fitness to plead, or insanity.
"[on the grounds of] diminished responsibility."