Making a character's personality consistent is an important factor. For example, having the character do something that completely contradicts their actions or beliefs up to that point, could seem out of place and jumpy to a reader, making the character less realistic.
Having distinctive character traits, that affect the way a character sees and interprets their surroundings, and thoughts, is going to be easier to maintain. Consider both notable physical traits such as birth marks, scars, freckles, piercings, a type of dress sense (E.g. quirky, gothic) tattoos Etc. as well as factors like strong religious or political beliefs, being family-orientated or intense feelings of anger. Giving inconsistent characters strong, distinctive traits prevents them from becoming an ordinary, wishy-washy person. For example, if a religious extremist is asked about their beliefs the answer is more definite, but if a non-religious person is asked the same thing it's harder to predict what would be said. Ie. ditinctive character traits give you character substance and body, which is easier to maintain than a lack of strong characteristaics. Also, when describing characters avoid a 'brown hair, blue eyes' description for 'scar above his eyebrow, swallow tattoo on his bicep' one. We notice the things that stand out about another person, not necessarily their hair and eye colour (unless they're particularly notable).
Consistency might be aided by a re-visit to your characters 'profile'. A list covering name, age, religious view, political views, motivations, economic position, location Etc. (you only need to include the distinctive traits) will familiarise yourself with the characters mindset.
And reading through your work so far will remind you of the kind of tone you started off the character with.
Do your characters have any distinctive traits?