The issue with "following the constitution and the law" is that any law has sort of component parts. What a Justice values always becomes in some way an application of the law. An easy example is the letter of the law vs the spirit of the law.

It gets murkier than that. Say you have a law, let's take a pretty straightforward one. "Killing someone is illegal except as a form of self defense to preserve one's own life". Seems pretty basic... But then you are not dealing with something that did happen (Killing someone) with something that only potentially could have happened but didn't (that person killing you). Every single component peice in that law requires a definition Killing someone becomes a modifiable base where whether you are guilty is based on how one reads the intention and capability of someone who isn't there to question because the defendant killed them. What if the defendant was legitimately in fear for their life but that fear was entirely misplaced? What if that person put themselves knowingly into greater danger to essentially unlock the ability to kill someone in self defense? The spirit of the law is often envoked to close loopholes the original writer of the law missed because they didn't forsee every possible circumstance but those things are not written in those laws... So what did the writer actually intend? What are the consequences the different possible active applications of the law have? How does it interact with other laws in the system and if it falls between two competing laws which law is more important to be upheld? What, counts as the guiding authority in application of the law.

A raft of precedent is usually used to see how other Justices handled similar issues in the past but precedent is more a guideline than a hard and fast rule. Every individual Justice in a system shapes the law. Appeals may push it higher in the system challenge the law against other laws but every level of law you have the same battle. Spirit vs letter of law, reaching consensus on defining every single word mentioned in the law , establishing what component peice of the law is the most important bit and then ordering the other components into priority and then either trying to squash other interpretations of the law as presented by the defense into that existing mold or realizing that the law was written without an important part that was likely not intended and changing precedent to accommodate leniency after the fact.