What does mantra shastra say about the role of Guru in mantra deeksha?

We have already seen the procedure for mantra sadhana in the absence of an able Guru. That is an exception.
As a thumb rule, the deeksha or initiation should be obtained from an able Guru.
Then only mantra will work.

यामले-
गुरोर्मुखान्महाविद्यां गृह्णीयात् पापनाशिनीम्।
तस्माद्यत्नाद्गुरुं कृत्वा मन्त्रसाधनमाचरेत्॥
You should put effort to find an able Guru.

What does the term Guru stand for?
यामले-
गकारः सिद्धिदः प्रोक्तो रेफः पापस्य दाहकः।
उकारः शम्भुरित्युक्तस्त्रिधात्मा गुरुरव्ययः॥
Gakara in Guru stands for someone who can give you siddhi, mantra siddhi.
He has that capability meaning, first of all, the Guru should have siddhi.

What is siddhi?
Someone with siddhi can apply the mantra for various results.
If the shastra says, if 1000 ahutis are offered in homa with pancha gavya with a certain mantra then it will cure diseases then, only if someone with siddhi in that mantra, if he does it, then only it will happen.
Otherwise not.
This is mantra siddhi.
The Guru should have siddhi in the particular mantra that he is initiating his shishya into.
Otherwise, it won't work.

Siddhida also means that the Guru should know the procedure to obtain siddhi.
The rules of the ten mantra samskaras, the procedure for purascharana, and all that.
Without following procedure, merely by chanting, mantra siddhi will not come.

The repha the र in Guru stands for papasya dahakaha.
The Guru is going to burn away all the papas in the shishya by giving mantra deeksha.
Siddhi will not come unless the bad karma of the past is burned away.
While the purascharana is like charging oneself with the mantra, at the same time, it burns away the papas also.
Removal of the bad and installing the good.
That's what the Guru is doing, initiating the shishya into this.
That's what र in Guru stands for.

And ukara in Guru; that stands for Shambhu, the Jagad-Guru.
Guru of the entire universe.
And the Guru in the human form represents that Jagad-Guru, the ultimate source of all knowledge.
Ukara reveals this link between the Guru in human form and the Jagad-Guru.
That it is the Jagad-Guru himself who is initiating through the Guru in human form.

What are the qualifications needed in a Guru?
सारसंग्रहे-
विशुद्धमातापितृको जितेन्द्रियः सर्वागमज्ञः परदुःखकातरः।
यथार्थवाग्वेदविदङ्गपारगः शान्तः कुलीनो गुरुरीरितो द्विजः॥
He should belong to a noble lineage.
He should have self-control control over the mind and senses.
He should be well versed in mantra shastra.
He should be kind in nature.
He should be honest.
He should be knowledgeable in the Vedas and the Vedangas.
Of course, this would mean that he should be a dwija, which the last term of this shloka itself specifies.
He should be peaceful.
And not only his family should be noble, his parents particularly should be noble.

You have to pay attention to all these before you identify someone as your Guru.
Not that mass mantra deeksha is being conducted; let me also go.

Another shloka adds some more qualities:
शान्तो दान्तः कुलीनश्च विनीतः शुद्धवेषवान्।
शुद्धाचारः सुप्रतिष्ठः शुचिर्दक्षः सुबुद्धिमान्॥
आश्रमी ध्याननिष्ठश्च मन्त्रतन्त्रविशारदः।
निग्रहानुग्रहे शक्तो गुरुरित्यभिधीयते॥
He should have tolerance.
He should be humble.
His clothes should be clean.
His behavior should be warm and pure.
He should be stable, not a fickle-minded person.
He should be observing personal hygiene.
He should be dexterous.
He should be intelligent.
He should be following the ashrma-dharma.
He should know the principles of meditation and be good at it.
And he should have the power both to bless and control.