When not hunting for food, trolls will sometimes sit and shout provocative and inflammatory messages at passers-by in their ancient and unfathomable tongue. If this happens it's best to ignore them.
If caught out in dry conditions (or seriously wounded) trolls aestivate. By secreting a cocoon and reducing their metabolism trolls can protect themselves and even regenerate. They can remain in this rock-like state indefinitely and will only emerge once recovered or more favourable environmental conditions return. Trolls can get quite comfy in stone form and sometimes refuse to re-awaken at all.
Note: Used this photoshop bubble brush effect for submerged troll sketch