Jamieson, Fausset and Brown on Acts 28:3

3. when Paul had gathered a bundle of sticks--"a quantity of dry sticks." The vigorous activity of Paul's character is observable in this comparatively trifling action [Webster and Wilkinson].

and laid them on the fire, there came a viper out of the heat--Having laid itself up among the sticks on the approach of the cold winter season, it had suddenly recovered from its torpor by the heat.

and fastened--its fangs.

on his hand--Vipers dart at their enemies sometimes several feet at a bound. They have now disappeared from Malta, owing to the change which cultivation has produced.