Question: Is it possible to use regular trainers/shoes (i.e., withou cleats) with clipless pedals?

A friend of mine mentioned he does this all the time. However, I cannot imagine how this would work. Surely the area of grip would be far too small to get any kind of purchase on the pedals.

We both have [Shimano PD-R540 SPD](https://bike.shimano.com/en-EU/product/component/tiagra-4700/PD-R540.html) pedals.

Answer: 
There is nothing to stop you from getting on your bike in regular trainers instead of shoes with cleats. you won't damage your pedals in any way as long as you don't have a rock lodged in the sole of your shoe. The problem, as far as I can see it, is that it is just not very comfortable due to the small surface area and flexible sole of the shoe. Your feet will feel the pressure localized into a very small area. The other issue is grip. There isn't a lot of traction since the body of the pedal wasn't designed with that in mind. I do my own mechanics out if my garage and will occasionally hop on the bike I'm working on to make sure my repair or adjustment is working properly. I have bikes with Ritchey Logic, Speedplay frogs, SpeedPlay Zeros, Shimano SPD and non-clipless pedals. I don't go and put on the proper shoes just to ride down the street and back while I test an adjustment. So, yes, you can ride without shoes with cleats but really only in a very limited way.
I have used normal office shoes on look road pedals (albeit shoes with relatively thick soles) This works fine for to/from work or lunchtime errands.

However I found that pedalling on the "underside" of the pedal was more comfortable in some thin-soled shoes. Not ideal but workable.

Another option, try clipping a plastic cleat in the pedal, an older and worn-out one would be perfect. Plus you can remove it when you want to ride with proper shoes.

I have look clipless on my road bike, but I put platforms back on my MTB for trips around town, mostly because our roads are still terrible, and the MTB has better brakes and more load-carrying capacity.

Cleats may not be best for you.