Getting seized spark plug out?

[Scarletpimpofnel]

Years ago I tried to change the plugs on an ancient daily that hadn't had plugs removed for many years. All were stiff but one was stuck fast so I left that as it was.

I need to change the plugs on another car in a week or so and am expecting one or more to also be seized in. What is the best way to get them out? Is it best to have engine hot or cold to get a seized plug out? Can I just use brute force til it shifts or will the thread get damaged?

I'm doing this simple job as a favour for someone and worried I'll end up trashing their head and car!

tia

[Scarletpimpofnel]

Thanks all, fingers crossed they'll come out easily.

When I was a youth it was almost a weekly task cleaning plugs (my old knackered cars usually were burning oil or running lean etc so plugs needed checking regularly). But nowadays many plugs don't need changing for 30,000 miles and garages don't seem to do it unless asked hence some cars can go way more than that with old plugs in which I think leads to stuck plugs (coupled with people not using the right lube when plugs are changed).