The Sign it occupies  Jupiter in Sagittarius, where it is most at home and most powerful is rather unbridled in it's effect, ready to "damn the torpedoes, full speed ahead"! (Bicyclist Lance Armstrong and writer Truman Capote) 

Jupiter in Capricorn, however, would present quite a different picture where the free-wheeling, shoot-from-the-hip, break out the confetti tendencies have a built-in speed regulator requiring us to exercise control and discipline before proceeding with determination.  (Aviator Charles Lindbergh and President Nixon)

The House it occupies  With Jupiter in the 1st we're inclined to be more than a little optimistic, opinionated, and extroverted personally.  (TV Evangelist Jim Bakker and General Napoleon Bonaparte I) 

Contrast this with Jupiter in the 12th, where often there is a distinct behind-the-scenes flavor to the influence of Jupiter -- even despite tremendous public popularity -- something remains hidden or private  (Magician David Copperfield and director Roman Polanski)

Its aspects to other planets and points  Jupiter conjunct the MC would suggest tremendous opportunities for career success, public exposure, and business expansion.  (Comedian Bill Cosby and Basketball star Shaquille O'Neal)

A Square aspect to Neptune might suggest a tendency toward over-embellishment or an extremely vivid imagination. (Gossip columnist Rona Barrett and Scottish author Sir Arthur Conan Doyle)  

With a Trine to Venus social popularity may come quite easily -- maybe even too easily, with a compelling desire for luxury or beauty.  (Actress/Pinup Jane Russell and TV evangelist Tammy Faye Bakker)
 

The House it Rules  It is ever so important to look at House rulerships when looking at the Horoscope.  In the case of Jupiter, we're looking for the House(s) with Sagittarius on the cusp.  These Houses will often have a demonstrable attachment to our large friend, Jupiter.  Sag on the 5th might suggest tremendous creativity and/or a little too much partying, risk-taking, or gambling. (Comedian George Carlin and actor Robert Downey Jr.) 

Sag on the 8th might represent an ongoing fascination with sexual matters, death, or the occult (Actress Mae West and Basketball star and actor Charlie Sheen)

Whether Jupiter is Rx  If Jupiter is Rx, it's rather like having an aspect with Saturn or placement in Capricorn.  There's a repressed expression of the expected tendencies.  Some astrologers (myself included) believe that a Retrograde Jupiter indicates the need to be alone with their thoughts at times, to have some private space available to reflect.  Also, some believe that the second half of the life has potential to be more rewarding than the first.  (Billionaire recluse Howard Hughes and "Silva Mind Control" Founder Jose Silva)