A strong suspicion of a crime and a risk of flight or tampering with evidence are required; the principle of proportionality applies. It is more commonly applied for catalogue offences.
These are measures applied instead of arrest, such as an obligation to sign in, residence restrictions or a travel ban. If they are breached, they may be converted into arrest.
In a criminal investigation, a lawyer protects your right to remain silent, checks the statement record and objects to procedural irregularities. While in custody you may request a free bar-appointed lawyer, and before giving a statement you may examine the file and take written notes of the questions.