Question: What is the mentality of a sincere disciple?
Srila Bhaktisiddhanta: Sincere disciples should be completely devoted to their guru. They should know their spiritual master to be as good as God and the only object of their love and devotion. “The spiritual master is my eternal master and I am his eternal servant.” This is how a disciple thinks. Service to the spiritual master is a disciple’s life, ornament, and means of survival. Disciples do not know anything except their spiritual master. They always think of their spiritual master while eating, sleeping, dreaming and serving. They are fully convinced that the spiritual master is the fully independent personality of godhead.

"Introduction to the Gaudiya-bhasya," a translation from Srila Bhaktisiddhanta's introduction to Caitanya-bhagavat, for which he provided extensive purports verse by verse.

"Those who are intelligent should study the concept of served and servant in order to understand the Supersoul, who is present within the external coverings of this visible world. As long as there is no concept of served and servant, the Supersoul will not be considered a single reality rather as a void. The varieties found within the one Reality indicate the different aspects of its real energies."

"Drashta (the seer) and drishya (the seen) - are we drashta or drishya? Whatever I have to say, my whole message is based on proper comprehension of this truth. But how few people have really understood this drig-drishya-vichara!"

Showering tears, Srila Sarasvati Thakura repeatedly spoke thus to Sripada Sundarananda Vidyavinoda while Sundarananda was preparing materials for Sarasvati Thakura's biography.

The topic of drashta and drishya is discussed in Bhagavad-gita, Srimad-Bhagavatam, and other shastras, but Srila Bhaktisiddhanta Sarasvati clarified and stressed the point perhaps more than any previous acharya:

Question: What are the faults of a vaisnava which when discussed
constitutes vaisnava-aparadha?
Answer: To discuss the faults that may arise by chance in a vaisnava is an offense. The main thing is that the discussion or even consideration of these faults in a vaisnava is an offense to the holy name, etc.

Casting false blame on a vaisnava and finding fault with a vaisnava for any of the following three reasons are offenses against the holy name:

1) Impurities that have come from the vaisnava's past — prag-utpanna.
2) Impurities that are perishing — ksysavasista.
3) Impure activities that take place accidentally — daivotpanna.

Here is a nice story from the life and teachings of Srila Bhaktisiddhanta Sarasvati Thakur:

Once some devotees were sent to establish a preaching center in Bengal. They worked day and night, equipping it as a beautiful temple.  When Srila Bhaktisiddhanta Sarasvati arrived to install the Deities, he was very pleased and asked about one brahmacari who had worked hard on the preparations.  The disciples told him, "Master, he became entangled with
a lady.  We rebuked him so much for his behavior that he fled from this place."