The simplest way to get a divorce is to explain that you have been living separate and apart for 1 year. If you and your spouse agree on the separation date, then you do not need to give any evidence to convince the judge that this was your separation date.
If you choose to apply for a divorce based on the other spouse having committed adultery or physical or mental cruelty, then the court will need to hear evidence that this happened during the marriage. In the case of adultery, the other spouse may admit that they committed adultery by signing a statement under oath or to say it under oath in court. If the spouse refuses to admit that it happened, then you will need to provide evidence to convince the court that the other spouse committed adultery.
Similarly, a judge will need to have evidence that the other spouse treated you with mental or physical cruelty in a way that it made living together intolerable. It would be uncommon for the other spouse to admit that they treated their spouse with physical or mental cruelty. If you chose that reason for the breakdown of the marriage, a judge would need to have a trial and you would need to provide enough evidence about the cruel behaviour to convince the judge that it made living together intolerable.
You should talk to a lawyer if you are thinking about using either cruelty or adultery as the basis for your divorce.
Remember, claiming or proving that adultery happened or that you were treated with mental or physical cruelty as the basis for your divorce will not affect what happens in your divorce. The only difference is that a judge may be able to make the divorce order before 12 months from the date of separation. But the divorce process may take longer because of added court time and document preparation that are needed to prove that adultery or cruelty happened.