reftex-cite-prompt-optional-args

reftex-cite-prompt-optional-args is a variable defined in `reftex-vars.el'.
Its value is

maybe

Documentation:
Non-nil means, prompt for empty optional arguments in cite macros.
When an entry in `reftex-cite-format' is given with square brackets to
indicate optional arguments (for example \cite[][]{%l}), RefTeX can
prompt for values. Possible values are:

nil Never prompt for optional arguments
t Always prompt
maybe Prompt only if `reftex-citation' was called with C-u prefix arg

Unnecessary empty optional arguments are removed before insertion into
the buffer. See `reftex-cite-cleanup-optional-args'.

You can customize this variable.