Franco,
If you are using CRM 2013 and above you can have Real Time workflow that can help you validate the process and cancel it if the fields are blank or don't meet certain criteria.
Making the field as mandatory might be a solution to you prompt user that should key in this field, but in some point this field is not necessary to set as mandatory, for example you have case: if Account industry is IT then you need to make sure that the salesperson key in the Number of Consultant, for example, by making it as mandatory.
If Account industry is Agriculture then the Number of Consultant field of course is not mandatory, instead, maybe you need another number like total size of field in hectare or who is their Farm Supplier, etc. And if Account industry is Food then you need Best Chef Name, for instance only.
So the conditions is can be various. Which you can use Business Rule or JavaScript as well.
But, if you already create your own process in the workflow, then you can utilize workflow for validation purpose.
Here are the useful example:
garethtuckercrm.com/.../crm-2013-new-features-using-real-time-workflow-for-validation-rules
nakedcrm.wordpress.com/.../using-workflows-to-validate-records-to-prevent-creates-updatesassign-and-deletes
It will help to prompt user based on certain conditions + cancel the workflow as well, meaning that not finishing the execution as completed.
But there are things you need to aware that the prompt only works for Real Time Workflow, otherwise it would only cancel the workflow without notification, and if you are not using CRM 2011 and above, you cannot have this feature, then it will only triggered in some condition after save, for example field change, created, etc based on workflow trigger point. And you cannot have condition based on child records.
Hope this helps.
Thank you.