AtD translation: Frank gazed brightly

[Mike Jing]

P647.39-648.3   The elfin sidekick was whistling softly through his teeth
the popular favorite “Daisy, Daisy,” which since Doc Holliday’s celebrated
rejoinder to Frank McLaury at the O.K. Corral had been sort of telegraphic
code among gun-handlers for Boot Hill. Frank gazed brightly, all but
sympathetically, into the eyes of his target, waiting for a fateful tell.

What does "brightly" mean here?