A PIP is for capability, not conduct. It suits someone who is trying but not reaching the standard: missed targets, repeated errors, slow output. Deliberate behaviour, such as rudeness, refusing instructions or breaking a rule, is misconduct and belongs in the disciplinary procedure. Putting a conduct issue on a PIP muddles both processes.
It should never be the first conversation. The employee should already have heard about the problem in one-to-ones or a review, with examples. A PIP that arrives as a surprise reads as a step towards dismissal, and the employee spends the period defending themselves instead of improving.
Rule out causes a plan cannot fix. Acas advises employers to find out why performance has dropped before acting. A health condition or disability, caring pressures, a missing piece of training, an unrealistic workload or broken tools will not be fixed by objectives. Each needs its own response, and some, like reasonable adjustments, are a legal duty.
Decide what success looks like before you start. The manager, HR and, ideally, the employee should all be able to say at the outset what result at the end of the plan would count as success. If nobody can, the objectives are not ready. Agree too what happens if the plan is only partly met: an extension, a different role, or the formal process.