When does __index metamethod get called?

[Chris <coderight@...>]

Example:

   tbl = {} 
   setmetatable(tbl, {__index = function(t,k) print("test") end})
   tbl[1] = 1
   tbl[2] = 2

Prints nothing... ??  

Also, is there any metamethod that you can hook to know when a value is
being set?  I know there is __newindex but that only appears to be
called for new values being inserted into the table.  I want to
hook both cases of new values and also setting existing values.

-- 
// Chris