When using font substitution (in this case substituting Helvetica for
Geneva), the font used in place of Geneva (Helvetica) will use the Geneva
spacing, not the Helvetica spacing. The point size, however, should be the
same (that is, Geneva 12 point will substitute Helvetica 12 point). Only
the spacing will be different.
Two versions of the same document (one using Geneva on screen and one using
Helvetica on screen) would look different when printed, even though you
print both documents in the same font (Helvetica).