Whenever I do a test flight with a company pilot (commonly much more experienced on type than I), I provide a preflight briefing. It goes something like: "These are the maneuvers we'll be flying today. I will continue to fly the plane no matter what. If you feel that you would like to take control, say I've got it, and I'll let go - and you'll have control, so be sure you want it then!". This comes from some earlier failings to brief well, and a little cockpit confusion. Recently, I gave this briefing to a very nice company pilot who'd been sent with the company Caravan floatplane for me to fly. After my giving the briefing, he calmly said: "No I won't be taking control, the airplane does not have dual controls.". Okay then, we both knew who was PIC for the entire flight!
Another peripheral note on this topic, is that if you are acting as PIC, and perhaps giving training within the scope of your privilege to do so, you may not be insured for your own life or injury. I found this out the hard way while finishing a float endorsement for a fellow in his amphibian. When we took off the runway, he was PIC. But, as he did not [yet] have a float rating, the instant we touched the keel to the water, I became PIC in the licensing sense. Then it went bad, and the insurer's entry point to the discussion about my insurance claim for injury was that PIC's are not insured, only passengers. It ended up less hard nosed than that, but it was a nuance I had not previously considered. So if you're PIC, and the other person flying has an accident, you may not have any insurance coverage for injury. This is worth understanding before you choose roles!