Because of their liking for gardens and particularly for compost heaps, grass snakes are often encountered in gardens. They are harmless to humans, though they can act quite aggressively if cornered, puffing up their bodies, hissing loudly and even lunging to make a strike with a closed mouth. However, that is not their only behaviour when they feel threatened. Sometimes they play dead, with mouth open and tongue hanging out. If they are actually caught, they will struggle violently and emit a foul-smelling substance from their anal glands.
Photo of grass snake playing dead (below) by Iain Leach.