For either diocesan or religious order, they usually expect the candidate to be involved in some form of ministry. But is there a certain preference as to which ministry they’re in? e.g. are altar servers favoured more than lectors? Does the candidate have to be a youth leader rather than a committed participant?
Either way, things will be tested after the discernment process during candidacy.
Other factors to consider include motivations. The most common excuses are thoughts of having no chance of getting married and no career. But what if even if these conditions hold and priesthood may actually bring out the best in that person? How about asking the hypothetical question whether they would still be interested in the priesthood even they have a good chance of pursuing marriage and a long term career.
The odd thing is, priests will never turn a blind eye to someone who has expressed initial interest, but then there is a long road during discernment and candidacy, before reaching postulancy.
At the same time, one should not partake in ministry solely for the purpose of candidacy. If they genuinely like it, it should be fine, and whether this holds perpetually will be tested in later stages.